1. Field of the Invention
The present invention relates generally to a crate for pallets of the type comprising metal tubes arranged in a lattice structure.
More particularly it relates to a pallet crate in which the metal tubes are intended to be welded at crossing points of the lattice and have respective mutually-facing flat surfaces at the crossing points.
2. Description of the Prior Art
As is known, crates of the type specified are used on pallets to contain and protect material which is supported by the pallet itself.
The pallets, generally of wood but also of metal, are arranged so as to be lifted and transported easily by suitable forks of lift trucks and the like.
For example a typical use for pallet crates is for liquid containers, generally comprising a tank of plastics material.
The thin walls of the tank flex under the weight of the liquid in the tank and bear against the inside of the crate which thus prevents the tank itself from collapsing.
Furthermore it is possible to stack several pallets one on top of another by virtue of the presence of the crate since the weight of the pallets is supported structurally by the crates.
In order to manufacture the crates it is necessary to weld together a plurality of metal tubes, for example of circular or square section, at numerous crossing points. This operation, which is usually complicated and expensive, is carried out by welding with solder or by so-called projection welding.
European Patent Application No. 0,370,307 describes a pallet crate comprising circular section tubes which each have a curved recess made by pinching at the crossing points.
At each crossing point the recesses face each other so that the tubes may be welded by projection welding, that is, by the passage of an electric current through the four points of contact thus formed, at which the resistance to the passage of an electric current is particularly high.
The manufacture of crates described above, although allowing a certain degree of automation, requires accurate control of the welding parameters, that is the current strength, the application of a force to compress one element against another and the period of time for which the current is passed.
This control complicates the manufacture of the pallet crates and also results in a considerable waste of energy.